1. Field
This disclosure generally relates to electrical power systems, and more particularly to fuel cell stack start-up systems.
2. Description of the Related Art
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst, disposed at the interfaces between the electrolyte and the electrodes, typically induces the desired electrochemical reactions at the electrodes. The location of the electrocatalyst generally defines the electrochemically active area.
One type of electrochemical fuel cell is the proton exchange membrane (PEM) fuel cell. PEM fuel cells generally employ a membrane electrode assembly (MEA) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two electrodes. Each electrode typically comprises a porous, electrically conductive substrate, such as carbon fiber paper or carbon cloth, which provides structural support to the membrane and serves as a fluid diffusion layer. The membrane is ion conductive, typically proton conductive, and acts both as a barrier for isolating the reactant streams from each other and as an electrical insulator between the two electrodes. A typical commercial PEM is a sulfonated perfluorocarbon membrane sold by E.I. du Pont de Nemours and Company under the trade designation NAFION®. The electrocatalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support).
In a fuel cell, an MEA is typically interposed between two separator plates that are substantially impermeable to the fuel stream. The plates typically act as current collectors and provide support for the MEA. In addition, the plates may have fuel channels formed therein and act as flow field plates providing access for the fuel streams to the respective porous electrodes and providing for the removal of reaction products formed during operation of the fuel cell.
In a fuel cell stack, a plurality of fuel cells are connected together, typically in series, to increase the overall output power of the assembly. In such an arrangement, one side of a given separator plate may serve as an anode flow field plate for one cell and the other side of the plate may serve as the cathode flow field plate for the adjacent cell. In this arrangement, the plates may be referred to as bipolar plates. Typically, a plurality of inlet ports, supply manifolds, exhaust manifolds and outlet ports are utilized to direct the fuel to the fuel channels in the flow field plates. The supply and exhaust manifolds may be internal manifolds, which extend through aligned openings formed in the flow field plates and MEAs, or may comprise external or edge manifolds, attached to the edges of the flow field plates.
A broad range of reactants, interchangeably referred to as fuel, can be used in PEM fuel cells. For example, the fuel may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
During normal operation of a PEM fuel cell, hydrogen in the fuel is electrochemically reduced on the anode side, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the membrane, to electrochemically react with oxygen in the oxidant on the cathode side. The electrons travel through an external circuit providing useable power and then react with the protons and oxygen on the cathode side to produce water.
One significant factor affecting the start-up process is the initial temperature of the fuel cells when the start-up process is initiated. When the fuel cells are relatively cold, the electrochemical reaction process is very inefficient. It is known to take measures to provide heat to the fuel cells during start-up to expedite the start-up process. For example, an auxiliary heater device may be used to provide heat to the fuel cells. It is also known to operate the fuel cells at a reduced voltage to internally generate heat within the fuel cells through relatively high power losses.
Conventional fuel cell stacks, and their associated individual fuel cells, operate at a relatively high minimum stack, cell, and/or the respective fuel cell voltage, during normal operating conditions. For example, in some automotive applications, a fuel cell stack provides a nominal output voltage of 250 volts (V) at 300 amps. Individual, serially connected fuel cells of the fuel cell stack output a nominal voltage of approximately 0.5 volts per fuel cell during normal operating conditions.
However, during a cold start-up process, especially sub-zero start-up, polarization curves are significantly less than the polarization curves provided from the fuel cells during normal operation. For example, a fuel cell stack may provide an output current of 100 amps at an operational stack voltage of 250 volts at cold temperatures and 200 amps at 250 volts at normal operational temperatures.
As the start-up process of the fuel cell stack proceeds, stack and individual fuel cell polarization curves rise from the above-described start-up polarization curves to the normal operating polarization curves. Accordingly, a period of time is required for the start-up process before sufficient voltage and current are available from the fuel cell stack for normal operating conditions.
However, various balance of plant (BOP) devices supporting operation of a fuel cell system are not always designed for operation at the reduced voltages provided by a fuel cell stack during start-up. An example of a BOP device is an oxidant supply device, for instance a blower, fan or air compressor that provides a nominal rate of airflow to the fuel cells when sourced, or powered, at the nominal voltage range during normal operating conditions. Another example is a coolant pump that circulates a coolant through the fuel cell stack at a nominal rate when sourced at the nominal voltage range. A further example is an anode recirculation pump that recirculates a fuel stream to the fuel cells at a nominal rate when sourced at the nominal voltage range. The above-described BOP devices are essential for fuel cell operation. Accordingly, during the start-up process before sufficient voltage and current are available from the fuel cell stack, these BOP devices are sourced from an auxiliary power supply, such as a battery, an ultracapacitor, and/or a relatively small combustion engine. However, such auxiliary power supplies may be limited in their output current and/or energy capacity especially when cold, thereby limiting the number of BOP devices and/or limiting the time that the BOP devices may be sourced.
Furthermore, during the start-up process before sufficient voltage and current are available from the fuel cell stack, other system loads may also require power from the auxiliary power supply. Otherwise, the other system loads must remain off until the fuel cell stack is able to provide sufficient voltage and current to source these system loads. For example, in some automotive applications, an electric passenger compartment heater is used to heat the passenger compartment. During start-up conditions, the passenger compartment heater cannot be operated unless otherwise sourced from the auxiliary power supply. Because of the high current drawn by the passenger compartment heater, it may not be practical to source the passenger compartment heater from a limited capacity auxiliary power supply. Accordingly, decreasing the period for start-up process of a cold fuel cell stack would be desirable.
Although there have been advances in the field, there remains a need in the art for increasing efficiency of the fuel cell stack start-up process. The present disclosure addresses these needs and provides further related advantages.